Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing methods is Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the Radio Frequency (RF) channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half, the RF channel may be used for UL. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many wireless communication systems, normally the communication between the base station and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally it may be on the order of milliseconds. For a given communication system the frame duration may be fixed. For example, the frame duration may be 10 milliseconds.
In a TDD wireless communication system, a frame may be divided into a DL subframe and a UL subframe. In TDD wireless communication systems, the communication from base station to the client terminal (DL) direction takes place during the DL subframe and the communication from client terminal to network (UL) direction takes place during UL subframe on the same RF channel.
Orthogonal Frequency Division Multiplexing (OFDM) systems such as 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system typically use Cyclic Prefix (CP) to combat inter-symbol interference and to maintain the subcarriers orthogonal to each other under a multipath fading propagation environment. The CP is a portion of the sample data that is copied from the tail part of an OFDM symbol to the beginning of the OFDM symbol as shown in FIG. 4. One or more OFDM symbols in sequence as shown in FIG. 4 are referred herein as OFDM signal.
Most wireless communication systems may employ some form of framing in the air interface. For example, 10 ms radio frames are used in the 3GPP LTE wireless communication system and each radio frame comprises 10 subframes as shown in FIG. 5. Each subframe in turn consists of two slots and each slot consists of 6 or 7 OFDM symbols depending on the type of CP used as shown in FIG. 5. In the 3GPP LTE wireless communication system, two different CP lengths are used and they are referred to as Normal CP and Extended CP. In wireless communication systems, normally the specific air interface frame structure repeats itself over certain periodicity.
The 3GPP LTE wireless communication system uses the following synchronization signals to assist the client terminal in achieving time and frequency synchronization as well as the detection of Physical layer Cell Identity (PCI):                Primary Synchronization Signal (PSS)        Secondary Synchronization Signal (SSS)        
The positions of the PSS and SSS within a frame are illustrated in FIG. 6 for FDD air-interface of 3GPP LTE wireless communication system. Note that the figure shows the position of the PSS and SSS for both the Normal CP and Extended CP. FIG. 7 illustrates the PSS and SSS positions for TDD air-interface of 3GPP LTE wireless communication system.
The different PSS and SSS are identified by different signal sequences used for transmission. Specifically, 504 PCI's are defined in 3GPP LTE wireless communication system specifications and they are organized into 168 groups with three identities in each group. The SSS sequence identifies the PCI group and PSS identifies the cell within a group.
The PSS sequences in frequency domain are length 63 Zadoff-Chu sequences extended with five zeros on each side and mapped to central 72 subcarriers as shown in FIG. 8. The Direct Current (DC) subcarrier is not used. In 3GPP LTE wireless communication system three different PSS sequences are used with Zadoff-Chu root indices 24, 29 and 34 corresponding to cell identity 0, 1 and 2 respectively within a PCI group. The exact PSS sequences are defined in the 3GPP LTE wireless communication system specification TS 36.211 “Physical Channels and Modulation.” The time domain PSS signal may be obtained by performing Inverse Discrete Fourier Transform (IDFT) of the frequency domain PSS signal.
The SSS sequences in frequency domain are generated by frequency interlacing of two length-31 M-sequences X and Y, each of which may take 31 different M-values. The SSS is extended with five zeros on each side and mapped to central 72 subcarriers as shown in FIG. 9. The DC subcarrier is not used. In 3GPP LTE wireless communication system, 168 valid combinations of X and Y are defined corresponding to 168 different PCI groups. The indices of particular values of X and Y are referred herein with m0 and m1. There are total of 31 different values possible for each of the indices m0 and m1. Therefore, the 168 valid PCI groups include m0 and m1 combinations that may have one of the two sequences common with other PCI groups. The time domain SSS signal may be obtained by performing IDFT of the frequency domain SSS signal.
In any wireless communication system, the signal transmitted by a cell may come across different propagation channel impairments before being received by a client terminal. The different propagation channel impairments include multipath propagation, scattering, shadowing, etc.
The multipath propagation refers to the reception of multiple delayed versions of the signal transmitted by a cell to a client terminal. The multipath propagation may result in inter-symbol interference in the time domain received signal which affects the orthogonality of the subcarriers and may cause the propagation channel to be a frequency selective channel. A multipath propagation channel model also referred to as power-delay profile is characterized by the power and delay of each path relative to the direct path.
Delay spread is the measure of the degree of multipath propagation in a wireless communication channel. It may be interpreted as a function of the difference between the time of arrival of the earliest significant multipath component and the time of arrival of the last multipath component. The Root Mean Square (RMS) delay spread is a metric to characterize the delay spread profile.
A discrete multipath time domain channel impulse response (CIR) is defined as follows:
                              h          ⁡                      (            n            )                          =                              ∑                          i              =              0                        I                    ⁢                                    α              i                        ⁢                          δ              ⁡                              (                                  n                  -                                      τ                    i                                                  )                                                                        (        1        )            where,
h(n) is the time domain CIR
ai is the complex amplitude of ith path
τi is the delay of ith path in number of samples
(l+1) is total number of taps in the power-delay profile
δ is the unit impulse response
The RMS delay spread (τrms) of a multipath profile is defined as follows:
                              τ          rms                =                                                            ∑                                  i                  =                  0                                I                            ⁢                                                                    (                                                                  τ                        i                                            -                                              τ                        _                                                              )                                    2                                ⁢                                                                                                a                      i                                                                            2                                                                                    ∑                                  i                  =                  0                                I                            ⁢                                                                                      a                    i                                                                    2                                                                        (        2        )            where,
τ is the mean excess delay and defined as
                              τ          _                =                                            ∑                              i                =                0                            I                        ⁢                                          τ                i                            ⁢                                                                                      a                    i                                                                    2                                                                        ∑                              i                =                0                            I                        ⁢                                                                            a                  i                                                            2                                                          (        3        )            
The 3GPP LTE wireless communication system uses the Cell specific Reference Signal (CRS) to assist the client terminals in estimating the channel and performing channel equalization i.e., to equalize the amplitude and phase distortions introduced by the frequency selective fading channel. The time domain positions of CRS within a subframe for normal CP and extended CP is illustrated in FIG. 10. The positions of the CRS in the time frequency grid of an OFDM symbol for normal CP and extended CP considering two transmit antenna ports are illustrated in FIG. 11.
In conventional methods, the channel estimation of non-CRS REs may be done using interpolation or filtering of CRS REs. The exact nature of interpolation or filtering may be a function of coherence bandwidth of the multipath power-delay profile. The coherence bandwidth of a channel, i.e., the frequency region over which a channel may be flat, is a function of the multipath power-delay profile. Thus the estimation of delay spread profile plays a crucial role in channel estimation and hence it may be useful to accurately identify the power-delay profile.
Conventional methods for delay spread profile estimation may use CRS in case of a 3GPP LTE wireless communication system. The high level block diagram for RMS delay spread estimation is illustrated in FIG. 12 and it involves the following three processes:                CIR generation (processing block 1202)        Number of valid path selection and computation of power of each path (processing block 1204)        RMS delay spread metric computation (processing block 1206)        
The CIR generation portion of delay spread estimation is illustrated in the block diagram 1300 contained in FIG. 13. At processing block 1302, the received time domain signal that includes CRS may be obtained after removing cyclic prefix based on the known OFDM symbol boundary. At processing block 1304, the frequency domain signal for a CRS OFDM symbol is obtained by performing Fast Fourier Transform (FFT) of the received time domain signal of that particular OFDM symbol. Then, at processing block 1306, the CRS RE's may be extracted from the received frequency domain signal of the CRS OFDM symbol based on the PCI input which may be obtained from cell search. At processing block 1308, the local replica of the CRS may be generated based on the a priori information about the modulating bits for CRS. The conjugate of the local replica may be multiplied with the extracted CRS to obtain the channel estimates of the CRS RE's in the processing block 1310. At processing block 1312, the demodulated CRSs of a particular CRS OFDM symbol are mapped to the corresponding frequency domain position thus constructing the channel frequency response (CFR) of a particular CRS OFDM symbol with accounting for the DC frequency subcarrier. The DC frequency subcarrier refers to the subcarrier at the zero frequency position. Transmission in DC or zero frequency may introduce distortion in the frequency spectrum due to local oscillator leakage. To avoid this impact, in 3GPP LTE wireless communication system DL DC subcarrier is unused. While generating the CFR of the CRS OFDM symbol, this needs to be accounted for, i.e., in the subcarrier corresponding to zero frequency, the value of zero needs to be inserted. The CIR may be generated by taking an Inverse FFT (IFFT) of the CFR in the processing block 1314. From the CIR, the valid path and corresponding power may be estimated, and from the estimated valid path, the RMS delay spread value may be computed.
The steps involved in valid path selection are illustrated in block diagram 1400 contained in FIG. 14. The output of the valid path selection is the power-delay profile. The steps involved in the RMS delay spread computation using the power-delay profile are illustrated in FIG. 15.
The use of CRS may be suitable for delay spread profile estimation for some scenarios such as deployment of higher channel bandwidths. In case of lower channel bandwidth deployments, the number of available CRS in one OFDM symbol may not be adequate for accurate delay spread estimation. A method and apparatus are disclosed that improve the delay spread estimation using other synchronization signals for lower channel bandwidth deployment scenarios.